The present invention relates to a cutting tool composed of coated cemented carbide based on tungsten carbide, which has high strength and high toughness and is particularly suited for cutting hardly machinable materials such as stainless steel.
As cemented carbide of widespread use for cutting metals, there has hitherto been known a WCxe2x80x94Co alloy comprising a hard phase containing tungsten carbide as a principal component and a binder phase made of a metal of the iron group such as cobalt, or an alloy obtained by adding carbides, nitrides or carbonitrides of metals of the groups 4a, 5a and 6a in the Periodic Table to the WCxe2x80x94Co alloy. In the latter, grains of the solid solution composed of both WC and carbides, nitrides or carbonitrides of metals of the groups 4a, 5a and 6a in the Periodic Table are added to the hard phase and binder phase.
These cemented carbides are now adopted, as a cutting tool, mostly for cutting cast iron and carbon steel. In recent years, they have also found application in cutting stainless steel. The stainless steel has been used in various fields because it is excellent in corrosion resistance, oxidation resistance and heat resistance. Thus, the processing amount of the stainless steel is increasing year by year.
However, the stainless steel is known as a typical hardly machinable material because of the occurrence of work hardening, low thermal conductivity, and high affinity with tool materials.
Among WC cemented carbides for cutting tool, cemented carbide that is classified into so-called M series in accordance with JIS B 4053 (1996) is generally used to cut stainless steel. In the M series, WCxe2x80x94TiCxe2x80x94Ta(Nb)Cxe2x80x94Co cemented carbide is mostly used, and TiC and Ta(Nb)C are added in a relatively small amount in order to impart toughness.
However, when stainless steel is cut even with a cutting tool made of conventionally cemented carbide of the M series, it is difficult to perform satisfactory cutting for a long time because the wear or the cutting tool is severe and thus its tool life expires in a short time.
In addition, the cutting resistance from the stainless steel surface subjected to work hardening during cutting can lead to severe damage of a primary boundary portion, resulting in a short tool life.
It is an object of the present invention to provide a cutting tool having a long tool life, which has improvements in wear resistance and plastic deformation resistance even when cutting hardly machinable materials such as stainless steel.
It is another object of the present invention to provide a method of cutting metal, such as hardly machinable materials (e.g., stainless steel).
The present inventor had intensive study of the foregoing problems and found the following novel fact. That is, in case that a region where a reduction ratio of Zr to the inside of a cemented carbide main body is smaller than a reduction ratio of other metals selected from the groups 4a, 5a, and 6a in the Periodic Table, is formed in vicinity of the surface of the cemented carbide main body, the resulting cemented carbide has excellent mechanical strength, as well as excellent wear resistance and plastic deformation resistance to the cutting of stainless steel.
A conventional cutting tool makes the machined surface of the material to be cut deteriorate by occurrence of chipping that is due presumably to deposition. Whereas in the present invention, the cemented carbide body itself can be strengthen and chipping resistance can also be improved by forming the above-mentioned region in the the surface portion of the cemented carbide main body.
Specifically, a cutting tool of the present invention comprises: a cemented carbide main body comprising a hard phase made up of WC and two or more selected from carbides, nitrides and carbonitrides at metals of the groups 4a, 5a and 6a in the Periodic Table, including Zr, and a binder phase composed of at least one metal of the iron group, and a coating layer formed on the surface of the cemented carbide main body. Especially, a region where a reduction ratio of Zr to the inside of the cemented carbide main body is smaller than a reduction ratio of other metals of the groups 4a, 5a and 6a in the Periodic Table is formed in the surface portion of the main body.
Further, according to the present invention, there is provided a method of cutting metal, such as hardly machinable materials (e.g., stainless steel), with the cutting tool as described above.
The other objects and advantages of the present invention will become apparent from the following detailed description.